1. Field of the Invention
The present invention relates to a method of manufacturing a silicon carbide semiconductor device having a plurality of regions with different impurity concentrations.
2. Description of the Background Art
An important problem in manufacturing a silicon carbide semiconductor device is to reduce a work period and a cost by reducing the number of process steps.
Generally, in a power semiconductor device, power of a relatively high voltage is handled, and therefore a breakdown voltage structure using the RESURF principle is often employed, as in a high breakdown voltage semiconductor device disclosed in Japanese Patent Application Laid-Open No. 8-306937 (1996), for example. In the RESURF structure, a p region and a p-well region formed around the p region and having a relatively low impurity concentration are provided, to thereby relieve an electric field at a terminal portion of a pn junction.
However, when this structure is employed, two p-type regions having different impurity concentrations need to be formed. For forming a plurality of regions having different impurity concentrations, a flow including the steps of (1) forming a mask that protects a region where no impurity is to be implanted, (2) implanting an impurity into an opening portion of the mask, and (3) removing the mask, is performed for each of the regions. Therefore, as the number of regions having different impurity implantation amounts increases, a work period and costs increase. Moreover, there is a problem that, in a case of an Al-ion implantation which requires high energy, a beam current is low and a ion-implantation time period is prolonged.
Accordingly, as a method of forming a plurality of regions having different impurity implantation amounts in a single mask step, for example, Japanese Patent Application Laid-Open No. 2004-260180 and Japanese Patent Application Laid-Open No. 11-121394 (1999) disclose a technique of forming a mask by arranging, at predetermined intervals, a plurality of unit masks each designed under consideration of a thermal diffusion length of implanted ion being diffused by a heat treatment.
Since a diffusion coefficient in silicon carbide is very small, an impurity which is ion-implanted into silicon carbide is not diffused by a heat treatment. Thus, it is impossible to form an impurity region by using the method disclosed in Japanese Patent Application Laid-Open No. 2004-260180 and Japanese Patent Application Laid-Open No. 11-121394 (1999). If impurity-implanted regions are formed in a silicon carbide semiconductor layer by performing an ion implantation into two opening portions which are separated from each other by a sufficiently large mask, two impurity regions are formed and these regions are not connected to each other due to diffusion even if a heat treatment is performed.